Auditory Perception and Olfactory Perception


Auditory pathway

  • Our auditory perception is also crucial and nuanced


  • Cochlea

    • Arranged like a spiral

    • Consists of membranes that move relatively to one another based on sound waves entering the ear.

    • It consists of cells that transduce sound energy into neuronal signals.

    • This motion stimulates the hair cells

    • Cells that have “hairs” called cilia

    • The movement of cilia causes the cells to generate a graded potential

      • Localized change in the membrane potential of a cell. The magnitude of the change directly corresponds to the strength or intensity of the stimulus and doesn’t follow an all-or-none law.

    • Hair cells then project to the spiral ganglion cells.

    • The axons of the spiral ganglion cell

      • Make up the auditory nerve

    • The stimulation of the hair cells can depend on the frequency.


Tonotopic map

  • Some cells respond ot higher frequency vs. lower frequency.

  • High frequency at the base of the cochlea.

  • Low frequency is located at the apex or tip of the cochlea.


1st step!

Brain stem

  • Information through the auditory nerve exits the ear.

  • First, the auditory nerve projects to the cochlear nucleus in the medulla.

  • Then, the information goes to the superior olivary nucleus also in the medulla.

  • It is both contralateral and ipsilateral

    • So, ears are compared much earlier, and it is not strictly contralateral like the visual system.



Pathway continues to the midbrain and thalamus


  • Information is then sent to the inferior colliculus in the midbrain,

  • Further, it goes to the medial geniculate nucleus of the thalamus

  • From the thalamus, information goes to the auditory cortex in the temporal lobe.

  • Like visual perception, top-down influences exist even in auditory perception.

    • Cognitive mechanism where the brain utilizes prior knowledge, context, and expectations to interpret incoming sensory information

   


Auditory cortex

  • Divided into regions

    • Core

    • Belt

    • Parabelt

  • Core contains the primary auditory cortex (A1), which receives input from the MGN

  • The belt receives input from the core, and the parabelt from the belt.

  • All the areas maintain a tonotopic map, which is based on frequency and not spatial location.

  • Cells exhibit a receptive field in a way that is dependent on the frequency preference

  • You can create tuning curves for these cells.

    • Belt and parabelt have broader preferences.

    • core is more pure tones; belt and parabelt are more complex,



Spatial localization based on sound input

  • This localization is computed by the auditory system as a whole and not just the ear.

  • The two ears receive different levels of sound information.

  • A sound wave traveling hits the two ears at different times since they are on opposite sides of the head.

  • This creates an interaural time difference

  • Not just the timind of the sound, but the intensity or loudness would be different too.

  • This creates an interaural intensity difference

  • This is an extremely precise system.


Outer ear’s role in sound localization

  • Not just the timing or intensity, but the shape of the sound wave can help.

  • Vertical dimensions are cause these shape differences.

  • This is greatly influenced by the outer ear


Brainstem’s role in sound localization

  • The brainstem has cells that are used to determine timing differences of sounds, known as coincidence detectors.

  • These cells are present in the superior olivary nucleus

    • First point where the information from the two ears can be compared.

    • These cells respond best to sounds that reach them at the same time


“What” and “where” pathways of the auditory cortex

  • Spatial information is processed on the posterior regions.

  • Nonspatial information is processed on the anterior regions.

    • Nonspatial information includes.


Audiovisual interactions

  • Visualprocessing is influenced by auditory information and vice versa.

  • There are spepcific areas of the brain involved in audiovisual interactiosn although convergence need not take that long to occue.

  • McGurk effect - Multisensory illusion

    • Hearing alone, seeing alone and hearing + seeing creates different experiences.




Olfactory System

  • Our olfactory senses are strong

  • It helps monitor the chemical composition of the environment, and it perceives it as distinct aromas.

  • When we sniff, the airborne, volatile chemicals are inhaled into our nasal cavity.

  • On the roof of our two nasal cavities lies the olfactory epithelium

  • OE is responsible for transducing neural signals.

  • Olfactory Epithelium

    • OE contains olfactory sensory cells or neurons arranged in a specific order and are odorant specific.

  • Olfactory bulb and primary olfactory cortex

    • These neurons send these specific signals to the olfactory bulb.

    • Then, these projects to the primary olfactory cortex

    • Further connections to the higher cortical limbic system allow us to have conscious experiences.


Other sensory experiences exist too

  • Gustation, that is, taste perception, exists and carefully maps cellular organization on the tongue and further.

  • Similarly, our somatosensory, aka tactile perception.